This invention relates generally to the prevention of breakage of marine propellers and their shafts; and more particularly to flow guide structure characterized as cooperating with a propeller to achieve this purpose, and additional advantages, as will appear.
Conventional propeller installations are characterized by tendencies for fatigue fracturing at blade root areas, and at drive shafts near the propellers. Analysis reveals that a propeller's axis of rotation is not parallel to the direction of water flow entering the propeller zone, and the thrust induced upon the blades is not the same at all phases of revolution. Thus, as a curved or angled blade rotates relatively downwardly, (for example at 9 o'clock rotation angle), the water loading thereon is different than the loading on the blade as it rotates relatively upwardly (for example at 3 o'clock rotation angle). The blade can as a result be cyclically unloaded and overloaded as it rotates, leading to fatigue and breakage of propeller blades and shafts, due to bending fatigue, and also cavitation.
Further, a condition called stern vibration or rumble is a common occurrence especially with the installation of two or four bladed propellers. In the case of a two bladed propeller, a full unloading and overloading of the propeller blades occur each 1/2 revolution, and for a four bladed propeller the same occurs each one fourth revolution. It is in general not practical to install a through hull propeller shaft, to extend axially parallel to the flow of water past the hull, so that the plane of propeller rotation is perpendicular to the water flow direction.